This makes gmnhg parse TOML/YAML/JSON/org-mode front matter in Hugo posts.
This also makes the library no longer render metadata in posts, removing
the API to do so. The metadata parsing code itself moves to
internal/gmnhg.
As the library no longer has a preset to include metadata in rendered
document, md2gmn will from now on silently discard front matter if it
encounters any.
Fixes#28. Unblocks #13.
gmnhg is the new program that generates a Gemini site from Hugo site
content. It reads its input from content/, static/, and layouts/gmnhg/.
Its output by default goes to output/.
More doc is available in the program doc header.
This also introduces a simple program, md2gmn, that is meant for testing
the renderer on Markdown files. Nonetheless, it can be used as
a standalone tool as well.
